Yes, checkers are getting flagged. Google's system now logs the IP and user-agent of any login attempt. When 800 fresh accounts all get accessed from the same checker IP within minutes, it triggers a compromised account review.
This forces the accounts into a verification hold, often requiring a phone number or QR code scan that you can't provide. The 700 verified result you're seeing is a false positive from the checker itself; it just confirms the login succeeded in that instant, but the accounts are immediately flagged and unusable.
To avoid this, you need to introduce human-like randomness. Use a checker that can rotate residential proxies and randomize delays between checks (60-120 seconds). Even better, create the accounts and let them sit for 48 hours before the first checker test. This mimics natural user behavior and reduces the automated signal.